A drilling operation for an oil or gas well usually involves drilling a borehole into a rock formation, lining the hole with steel pipe (also referred to as “casing”), and then pumping cement slurry into the annular space between the steel pipe and the borehole wall. Cementing the casing in place serves to reinforce the structural integrity and mechanical stability of the borehole, as well as to achieve “zonal isolation” by preventing fluid flow along the outside of the casing. When zonal isolation is lost, fluids from high-pressured formation regions can pass through the borehole to invade lower-pressured formation regions or exit the borehole in a mixture with other fluids, thereby potentially diluting the production fluid (e.g., oil or gas), damaging the hydrocarbon reservoir, contaminating aquifers, or otherwise causing environmental harm and/or reducing the profitability of the well. Accordingly, it is desirable that the cement completely fill the annular space between the casing and the formation.
A complete cement sheath is, however, not easy to accomplish. For example, irregularities in the borehole shape may render the requisite amount of cement unpredictable; voids, fractures, and pores in the surrounding formation may allow cement to escape the borehole; and fluids trapped in the annular space may be hard to replace with cement slurry. To detect such problems early on and thereby facilitate their correction or mitigation, the quality of the cement sheath is routinely assessed based on measurements taken with a suitable logging tool during a wireline logging operation.
A similar problem arises with respect to gravel packing. To prepare the well for production of hydrocarbons, production tubing is usually run through the well, inside the casing, into the hydrocarbon reservoir. The lower portion of the production tubing may have holes that admit oil and gas. The perforated section of the tubing may be surrounded by a screen and gravel pack designed to prevent sand infiltration into the production tubing. For proper functioning, the gravel pack desirably fills the annulus between the production tubing (or screen therearound) and borehole casing completely. In order to ensure the quality of a gravel pack and trigger remediating action if voids are detected therein, the gravel pack quality is likewise routinely measured, using similar tools and techniques as are employed for cement-quality assessment.
Logging tools for assessing cement and/or gravel-pack quality include, for example, sonic/ultrasonic, gamma-ray, neutron, and pulsed neutron tools. Measurements using pulsed neutron tools are conventionally based on the activation of silicon nuclei (i.e., the generation of unstable silicon isotopes) in the cement or gravel by high-energy neutrons, and spectral measurements of the gamma rays created during the decay of these isotopes. These measurements are generally performed in a logging pass separate from that used to evaluate the upward), adding time and cost to the logging operation. Certain alternative techniques, such as acoustic/ultrasonic measurements, suffer from low radial depth and may sometimes be sensitive only to the casing/cement contact. Accordingly, alternative methods for assessing cement or gravel-pack quality are needed.